1. Field of the Invention
The invention relates to rotary drag-type drill bits for use in drilling holes in subsurface formations and of the kind comprising a bit body having a leading face formed with a central depression which is generally coaxial with the axis of rotation of the bit, and a plurality of cutters mounted on the leading face of the bit, both within and outside said central depression. One common form of bit has a shank for connection to a drill string, a plurality of circumferentially spaced blades on the bit body extending outwardly from the central axis of rotation of the bit, and defining said leading face of the bit, a plurality of cutters being mounted along each blade. The central depression is then defined by the shapes of the inner portions of the blades. A passage in the bit body supplies drilling fluid to nozzles in the surface of the bit, between the blades, for cleaning and cooling the cutters.
The central depression in the leading face of the bit is commonly referred to as the "cone", although it is not necessarily conical in the strict geometrical sense. For example, the internal surface at the apex of the cone may be smoothly rounded, and the outer circumference of the cone is usually also smoothly rounded in section so as to blend smoothly with the rest of the leading face of the bit. For convenience, however, the central depression in the drill bit will be referred to herein as the "cone", in accordance with the conventional terminology in the drill bit industry.
The volume enclosed between the interior surface of the cone and an imaginary flat plane extending tangentially across the mouth of the cone is referred to as the "cone volume". Conveniently, the interior surface of the cone maybe regarded as being defined by the cutting profile within the cone or, for simplicity, by a smooth imaginary surface extending tangentially across the tips of the cutters mounted within the cone.
2. Description of Related Art
As is well known in the subsurface drilling industry, it is usual for drill bits to demonstrate a tendency to deviate from a straight path while drilling, by turning, or "walking", to the side particularly when the bit is drilling in a direction which is substantially horizontal or is at a comparatively small angle to the horizontal. Generally speaking, drag-type drill bits have a tendency to walk to the left, as a result of the fixed cutters generating a left-hand reactive torque during drilling.
It is common practice to use this tendency of a drill bit to walk as a parameter in controlling the direction of drilling. However, in order that this may be done effectively it is necessary to know the average walk rate of a drill bit, expressed as angular deviation over distance. Hitherto, it has not been possible to predict the walk rate of a particular drill bit design since the manner in which the various constructional characteristics of a drag-type drill bit affect walk rate have not been fully understood or quantified. It is has therefore only been possible to determine the walk rate of a particular design of bit by obtaining data from use of the bit in the field.
It would therefore be desirable to be able to predict the walk rate from data relating to the structure of the drill bit. This would then enable the performance of the drill bit to be predicted, insofar as it affects the direction of drilling, before the bit is actually operated downhole. Also, it would enable bit designs to be created or modified in a manner to give a desired walk rate. For example, under some drilling conditions it may be desirable for the walk rate of the drill bit to be at a minimum.
The present invention is based on the discovery that, in a drill bit of the kind first referred to and formed with a central depression or cone, the walk rate of the bit can be related to certain structural characteristics of the bit, thus enabling the walk rate of the bit to be predicted from such structural characteristics. In particular it has been found that the walk rate of such a drill bit is related to the product of the cone volume and the percentage of the total torque generated by the cutters on the bit which is attributable to the cutters mounted within the cone.